Re: [PATCH 2/2] target: iscsi: fix a race condition when aborting a task

[Mike Christie <michael.christie@oracle.com>]

On 10/27/20 8:49 AM, Maurizio Lombardi wrote:
> Hello Mike,
> 
> Dne 22. 10. 20 v 4:42 Mike Christie napsal(a):
>> If we free the cmd from the abort path, then for your conn stop plus abort race case, could we do:
>>
>> 1. thread1 runs iscsit_release_commands_from_conn and sets CMD_T_FABRIC_STOP.
>> 2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It then returns from the aborted_task callout and we finish target_handle_abort and do:
>>
>> target_handle_abort -> transport_cmd_check_stop_to_fabric -> lio_check_stop_free -> target_put_sess_cmd
>>
>> The cmd is now freed.
>> 3. thread1 now finishes iscsit_release_commands_from_conn and runs iscsit_free_cmd while accessing a command we just released.
>>
>>
> 
> Thanks for the review!
> 
> There are definitely some problems with task aborts and commands' refcounting *
> but this is a different bug than the one this patch is trying to solve (a race to list_del_init());
> unless you are saying that abort tasks should never be executed when the connection 
> is going down and we have to prevent such cases from happening at all.

Yeah, I think if we prevent the race then we fix the refcount issue and your issue.
Here is a patch that is only compile tested:

From 209709bcedd9a6ce6003e6bb86f3ebf547dca6af Mon Sep 17 00:00:00 2001
From: Mike Christie <michael.christie@oracle.com>
Date: Tue, 27 Oct 2020 12:30:53 -0500
Subject: [PATCH] iscsi target: fix cmd abort vs fabric stop race

The abort and cmd stop paths can race where:

1. thread1 runs iscsit_release_commands_from_conn and sets
CMD_T_FABRIC_STOP.
2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It
then returns from the aborted_task callout and we finish
target_handle_abort and do:

target_handle_abort -> transport_cmd_check_stop_to_fabric ->
lio_check_stop_free -> target_put_sess_cmd

The cmd is now freed.
3. thread1 now finishes iscsit_release_commands_from_conn and runs
iscsit_free_cmd while accessing a command we just released.

In __target_check_io_state we check for CMD_T_FABRIC_STOP and set the
CMD_T_ABORTED if the driver is not cleaning up the cmd because of
a session shutdown. However, iscsit_release_commands_from_conn only
sets the CMD_T_FABRIC_STOP and does not check to see if the abort path
has claimed completion ownership of the command.

This adds a check in iscsit_release_commands_from_conn so only the
abort or fabric stop path cleanup the command.
---
 drivers/target/iscsi/iscsi_target.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index f77e5ee..85027d3 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -483,8 +483,7 @@ int iscsit_queue_rsp(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
 void iscsit_aborted_task(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
 {
 	spin_lock_bh(&conn->cmd_lock);
-	if (!list_empty(&cmd->i_conn_node) &&
-	    !(cmd->se_cmd.transport_state & CMD_T_FABRIC_STOP))
+	if (!list_empty(&cmd->i_conn_node))
 		list_del_init(&cmd->i_conn_node);
 	spin_unlock_bh(&conn->cmd_lock);
 
@@ -4088,6 +4087,16 @@ static void iscsit_release_commands_from_conn(struct iscsi_conn *conn)
 
 		if (se_cmd->se_tfo != NULL) {
 			spin_lock_irq(&se_cmd->t_state_lock);
+			if (se_cmd->transport_state & CMD_T_ABORTED) {
+				/*
+				 * LIO's abort path owns the cleanup for this,
+				 * so put it back on the list and let
+				 * aborted_task handle it.
+				 */
+				list_add_tail(&cmd->i_conn_node,
+					      &conn->conn_cmd_list);
+				continue;
+			}
 			se_cmd->transport_state |= CMD_T_FABRIC_STOP;
 			spin_unlock_irq(&se_cmd->t_state_lock);
 		}
-- 
1.8.3.1